When we drive somewhere new, we navigate by referring to a two-dimensional map that accounts for distances only on a horizontal plane. According to research published online in August in Nature Neuroscience, the mammalian brain seems to do the same, collapsing the world into a flat plane even as the animal skitters up trees and slips deep into burrows.

“Our subjective sense that our map is three-dimensional is illusory,” says Kathryn Jeffery, a behavioral neuroscientist at University College London who led the research. Jeffery studies a collection of neurons in and around the rat hippo­campus that build an internal representation of space. As the animal travels, these neurons, called grid cells and place cells, respond uniquely to distance, turning on and off in a way that measures how far the animal has moved in a particular direction.

Past research has focused on how these cartographic cells encode two-dimensional space. Jeffery and her col­leagues decided to look at how they respond to changes in altitude. To do this, they enticed rats to climb up a spiral staircase while the scientists collected electrical recordings from single cells. The firing pattern encoded very little in­formation about height.

The finding adds evidence for the hypothesis that the brain keeps track of our location on a flat plane, which is defined by the way the body is oriented. If a squirrel, say, is running along the ground, then scampers straight up a tree, its internal two-dimensional map simply shifts from the horizontal plane to the vertical. Astronauts are some of the few humans to de­scribe this experience: when they move in space to “stand” on a ceiling, they report a moment of disorientation before their mental map flips so they feel right side up again.

Researchers do not know yet whether other areas of the brain encode altitude or whether mammals simply do not need that information to survive. “Maybe an animal has a mosaic of maps, each fragment of which is flat but which can be oriented in the way that’s appropriate,” Jeffery speculates. Or maybe in our head, the world is simply flat.